No heat, due to a dirty flame sensor. Cleaned flame sensor, and heat works. Inspected system. Heat exchanger has some heavy hot spots and it’s compromised, evaporator coil is covered by biological growth, single capacitor is weak, and blower wheel dirty. Also, outside dual capacitor is very weak (reading 15 on a 45 microfarad), and no flood protection in the drain line. System is working, but air quality in the house is severe contaminated, less air flow and heat exchanger compromised. I didn’t find ny crack, at this time, but it can occur anytime. Sent estimates for potential repairs (not recommended), but the best option, due to the age (13 years old) and conditions of the system (heat exchanger compromised, evaporator coil covered biological growth,etc) would be to upgrade the system, with 10 years warranty and WiFi thermostat. Based on our findings today, it might be time to think about getting a brand new system.